This invention generally relates to ventable seals for pressurized containers and, more particularly, to ventable seals for electrochemical cells.
Electrochemical cells, such as cylindrical alkaline electrochemical cells, employ two electrochemically active materials and an aqueous electrolyte. The electrochemically active materials are typically manganese dioxide and zinc. These materials are conventionally housed in a cylindrical elongated container that is open on one end so that the electrochemically active materials and electrolyte can be inserted therein during the cell manufacturing process. A closure assembly that incorporates a disc shaped polymeric seal member, a rigid inner cover and an elongated metallic current collector that projects through the center of the seal member, closes the open end of the container. The seal member usually includes a hub, which surrounds the collector, and a thin diaphragm integrally molded into the central region of the seal body. The function of the diaphragm is to rupture and release gas from within the cell when the internal pressure becomes too high. The collector provides a conductive path between the zinc and one of the cell's terminal covers which is located on an end of the cell.
Manufacturers of electrochemical batteries constantly strive to improve the performance of their products in a wide variety of battery powered devices. A key objective of most battery manufacturers is to improve the amount of time that a battery will power a device. One strategy to accomplish this objective is to minimize the volume occupied by the cell's nonelectrochemically active components, such as the cell's vent assembly, thereby leaving more volume within the cell available for the electrochemically active components. In recent years, vent assemblies have been made more volume efficient by redesigning them to require fewer components and yet operate more reliably than previously known vent assemblies. As the volume occupied by the vent assembly has decreased, the problem of a vented seal “resealing” against another component of the vent assembly has become more apparent. This problem can occur in primary batteries, also known as nonrechargeable batteries, if they are charged by other batteries. While most batteries are used in a conventional manner, a small percentage of batteries are exposed to an abusive condition such as when a primary battery is inadvertently placed in a device such that the battery's positive and negative terminals are reversed. This problem could occur, for example, in some commercially available flashlights that are powered by eight AA size alkaline batteries. Typically, the batteries are inserted into two elongated chambers containing four batteries each. According to the instructions that accompany the flashlight, the batteries must be inserted “in series,” which means that the positive terminal of one battery contacts the negative terminal of an adjoining battery. Unfortunately, some consumers fail to comply with the instructions that are normally included with the device and accidentally insert one of the cells backwards. If this occurs with the second or third battery in a cavity containing four batteries, then the cell inserted backwards will be charged by one or more of the properly inserted cells when the light's switch is moved to the “on” position. Charging the battery that has been inserted backwards generates significant quantities of heat and gas within the battery. In some situations, the external temperature of the battery could exceed 65° C. The increase in temperature increases the pressure within the cell. In addition to increasing the temperature of the battery, the chemical reactions that take place during the charging of the cell generate gas that substantially increases pressure within the cell. The simultaneous production of gas and increase in temperature causes the polymeric seal, which is typically made of nylon, to become soft and lose some of its structural rigidity. The thin ventable portion of the seal may become elongated due to both the heating of the nylon and the increase in internal pressure. Consequently, when the softened and distorted seal ruptures in response to the pressure buildup, an initial quantity of gas may escape from within the cell but the tear in the ruptured seal may be resealed when the softened ruptured seal contacts the smooth inner surface of the terminal cover and reseals against the terminal cover. This problem is particularly acute with low volume vent assemblies wherein the distance between the seal member and other components, such as the cell's cover, is very small. If the ruptured seal does reseal against the cover and the cell continues to generate gas, the cell may eventually experience a crimp release wherein the crimped connection between the seal and container is broken and the vent assembly is forcefully ejected from the container.
As disclosed in U.S. Pat. No. 6,270,919 B1, previous attempts to prevent resealing of a ruptured seal body have included modifying a seal's inner disc portion to include ribs. The ribs are designed to maintain the opening in a ruptured seal body thereby preventing resealing of the vent mechanism. However, while the inclusion of ribs in the seal's diaphragm is helpful in preventing resealing in most cells, some seals with the ribs incorporated therein may be deformed when exposed to the heat generated during charging such that the ribs cannot maintain the opening in the seal after it has ruptured.
In a seal embodiment disclosed in U.S. Pat. No. 6,312,850 B1, vertical grooves were placed in the surface of a compression member that forms a part of the seal assembly. The grooves are designed to prevent resealing of a vented seal's diaphragm. The grooves create channels that allow the gas to vent and thereby prevent resealing of the vented diaphragm. While this embodiment does prevent resealing of the diaphragm against the surface of the hub, the grooves in the compression member cannot prevent resealing of the torn diaphragm against the interior surface of the cell's terminal cover.
U.S. Pat. No. 6,270,918 B1 discloses a seal assembly that utilizes a seal member and an inner cover with openings incorporated therein. The seal member directly abuts the inner cover. An outer cover is secured to the seal assembly and forms a contact terminal of the cell. If the cell's pressure increases beyond an acceptable limit, the seal member ruptures thereby allowing gas to escape through the openings in both the inner cover and contact terminal. While the described seal assembly does safely vent an electrochemical cell, the seal assembly relies upon the use of the inner cover. Unfortunately, the inner cover occupies space within the cell that could be better used to store more of the cell's electrochemically active materials.
Therefore, there exists a need for an inexpensive and simple to manufacture polymeric vent assembly that occupies a minimum amount of volume within the cell and can reliably prevent resealing of a vented seal against the electrochemical cell's terminal cover.